This invention relates to electrolytes which function as a source of alkali metal ions for providing ionic mobility and conductivity. The invention more particularly relates to electrolytic cells where such electrolytes function as an ionically conductive path between electrodes.
Electrolytes are an essential member of an electrolytic cell or battery. In one arrangement, a battery or cell comprises an intermediate separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for making such cells in which the ion source electrode is a lithium compound or other material capable of intercalating lithium ions, and where an electrode separator membrane comprises a polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility.
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on separator structures or membranes which physically contained a measure of fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber, filter paper or cloth to microporous polyolefin film or nonwoven organic or inorganic fabric have been saturated with solutions of an inorganic lithium compound, such as LiClO4, LIPF6, or LiBF4, in an organic solvent to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes has effectively provided the necessary Li+ ion mobility at conductivities in the range of about 10xe2x88x923 S/cm.
Lithium metal anodes cause dendrite formation during charging cycles which eventually leads to internal cell short-circuiting. Some success has been achieved in combatting this problem through the use of lithium-ion cells in which both electrodes comprise intercalation materials, such as lithiated metal oxide and carbon (U.S. Pat. No. 5,196,279), thereby eliminating the lithium metal which promotes the deleterious dendrite growth. Another approach to controlling the dendrite problem has been the use of continuous films or bodies of polymeric materials which provide little or no continuous free path of low viscosity fluid in which the lithium dendrite may propagate. These materials may comprise polymers, e.g., poly (alkylene oxide), which are enhanced in ionic conductivity by the incorporation of a salt, typically a lithium salt such as LiClO4, LiPF6, or the like. A range of practical ionic conductivity, i.e., over about 10xe2x88x925 to 10xe2x88x923 S/cm, was only attainable with these polymer compositions at well above room temperature, however. (U.S. Pat. Nos. 5,009,970 and 5,041,346.)
More specifically, electrolytic cells containing an anode, a cathode, and a solid, solvent-containing electrolyte incorporating an inorganic ion salt were referred to as xe2x80x9csolid batteriesxe2x80x9d. (U.S. Pat. No. 5,411,820). These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., xe2x80x9cliquid batteriesxe2x80x9d) including improved safety factors. Despite their advantages, the manufacture of these solid batteries requires careful process control to minimize the formation of impurities. Solid batteries employ a solid electrolyte matrix interposed between a cathode and an anode. The inorganic matrix may be non-polymeric [e.g., xcex2-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283.
Examples of solvents known in the art are propylene carbonate, ethylene carbonate, xcex3-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, diethoxyethane, and the like. These are examples of aprotic, polar solvents.
More recently, a highly favored electrolyte/separator film is prepared from a copolymer of vinylidene fluoride and hexafluoropropylene. Methods for making such films for cell electrodes and electrolyte/separator layers are described in U.S. Pat. Nos. 5,418,091; 5,460,904; and 5,456,000 assigned to Bell Communications Research, each of which is incorporated herein by reference in its entirety. A flexible polymeric film useful as an interelectrode separator or electrolyte member in electrolytic devices, such as rechargeable batteries, comprises a copolymer of vinylidene fluoride with 2 to 25% hexafluoropropylene. The film may be cast or formed as a self-supporting layer retaining about 20% to 70% of a high-boiling solvent or solvent mixture comprising such solvents as ethylene carbonate or propylene carbonate. The film may be used in such form or after leaching of the retained solvent with a film-inert low-boiling solvent to provide a separator member into which a solution of electrolytic salt is subsequently imbibed to displace retained solvent or replace solvent previously leached from the polymeric matrix.
Regardless of which technique is used in preparing an electrolyte/separator, problems occur including operability of the electrolyte in a relatively narrow temperature range; loss of effectiveness of the electrolyte; and electrolyte degradation. There is presently no effective means to maintain the serviceability of the electrolyte over a broad temperature range, particularly low temperature.
In view of the above, it can be seen that it is desirable to have an improved electrolyte which is operable over a relatively broad temperature range, including low temperature, and which maintains cell capacity in a variety of electrolyte/separator configurations, including those described above as exemplary.
The present invention provides a novel electrolyte solvent which is usable with a variety of carbonaceous and metal oxide electrode active materials, providing improved performance over a broad temperature range, and which is stabilized to maintain cell capacity over a number of cycles. The electrolyte includes a specifically selected class of solvents, and solvent combinations using such new solvents. The new solvents, when used as co-solvents, enhance the operable temperature range of the solvent mixture. The solvents of the invention are esters, generally characterized with lower melting points and higher boiling points compared to the range observed for commonly used solvents, such as dimethyl carbonate or diethyl carbonate. The novel, ester solvents of the invention have further lower melting points and higher boiling points than conventional solvents. The solvents are useful as both high and low temperature solvents but are particularly useful for low temperature applications such as start, light, ignition (SLI). The compounds usable as solvents according to the invention are compounds represented by the general formula Rxe2x80x2 COORxe2x80x3 (alkyl aliphatic ester) where Rxe2x80x2 and Rxe2x80x3 are each independently selected from the group consisting of ethyl and propyl.
In one embodiment, the ester represented by the general formula is included in a solvent mixture which also comprises ethylene carbonate (EC) and propylene carbonate (PC). In one embodiment, the combined amount of the EC and PC is greater, on a weight basis, than the amount of the ester of the formula stated above.
In another embodiment, the solvent mixture further comprises one or more other organic solvents along with the ester, and with the EC and/or PC ester mixture. When such other additional organic solvent or solvents is included in the mixture, it is preferred that such solvent be selected from the group of carbonates; lactones; propionates; five member hetercyclic ring compounds; and organic solvent compounds having a low alkyl (1-4 carbon) group connected through an oxygen to a carbon, and comprising C/O/C bonds.
One preferred solvent mixture comprises EC, or EC and PC; DMC; and the ester compound Rxe2x80x2 COORxe2x80x3 of the invention. A preferred combination is EC/DMC/Rxe2x80x2 COORxe2x80x3 or EC/DMC/EP/PC in weight ratios as follows: EC/DMC/EP at about 25:40:35 and at about 3:5:2 and EC/DMC/EP/PC at about 58:29:12:1.
Advantageously, the solvent ester of the invention is usable with a variety of cell electrode active materials including lithium, transition metal oxide compounds such as LiMn2O4, LiNiO2, LiCoO2, LiNiVO4, and LiCoNiO2. It is most preferred that the electrode active material be lithium manganese oxide represented by the nominal general formula. Li1+xMn2xe2x88x92xO4 (xe2x88x920.2xe2x89xa6xxe2x89xa60.2)
Advantageously, the ester solvent of the invention is usable with graphite active material consisting of particles which have an interlayer distance spacing of 002 planes as determined by X-ray diffraction of 0.33 to 0.34 nanometers; a crystallite size in the direction of C-axis (Lc) being greater than about 20 nanometers and less than about 2000 nanometers; and at least 90% by weight of the graphite particles having a size less than about 60 microns. It is most preferred that the graphite particles have a BET surface area greater than about 0.3 meters square per gram and up to about 35 meters square per gram.
In the case where one or more additional organic solvents is used in a solvent mixture along with the ester, the added solvents are preferably organic solvents having a boiling point of about 80xc2x0 C. to about 300xc2x0 C. and are capable of forming a solute with lithium salts. Preferably the added solvents are also characterized by being aprotic, polar solvents. Preferred additional organic solvents are ethylene carbonate (EC), dimethyl carbonate (DMC), propylene carbonate (PC). The relative amounts of the added solvents and the ester compound may vary so long as the ester of the invention is present. One particularly useful combination is a solvent mixture comprising EC/PC/DMC/Rxe2x80x2 COORxe2x80x3, where Rxe2x80x2 and Rxe2x80x3 are each independently selected from the group consisting of ethyl and propyl. Except for the present invention, there is not known to be the use of solvent combinations comprising EC/PC/Rxe2x80x2 COORxe2x80x3.
Advantageously, the solvent of the present invention exhibits good performance even with carbonaceous electrode active materials and with transition metal active electrode materials. These materials are known to show poor performance when used with more conventional organic solvents.
Objects, features and advantages of the invention include an improved electrochemical cell or battery having good charging and discharging characteristics; a large discharge capacity; good integrity over a long life cycle; and operability over a large temperature range and particularly relatively low temperature; and which is stable with respect to carbonaceous and graphitic electrode active material and stable with respect to metal oxide electrode material.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments, claims, and accompanying drawings.